Constant amplitude sweep generator



July 14, 1953 E. F. WELLER, JR., ETAL 2,645,715

CONSTANT AMPLTUDE SWEEP GENERATOR Filed Feb. 12, 1951 l i Patented July 1.4,Y

T 2,645,715 .CONSTANT AMPLITUDE SWEEP GENERATOR T Edward F. Weller, Jr., Detroit, Mich., and Richard G. Doyen, Milwaukee, Wis.,V assignors `to General MotorsV CorporatiomgDetroit, Mich., a cor-` porationV of Delaware Application February 12, 1951, Serial No. 210,515

This invention relates to asweep generator, and more particularly to"a variable frequency constant amplitude sweep generator.

` Inmaking certaintests upon avehicle engine, as "for example,V tests of the ignition system or of detonation characteristics, it is often desirable to make the tests under road operating conditions rather than under bench or laboratory condi- T tions. Often symptoms of defective or abnormal tion is often provided by means of an oscilloscope having a sweep which is synchronized with engine speed. 'Commercially obtainable oscilloscopes are not so constructed that the sweep will be of constant length throughout the range of engine speeds, the variation in sweep length being as great as ninety percent between idling speed and high speed. Consequently,`with test equipment of this type, an auxiliary sweep generating circuit preferably is provided for the oscilloscope, this circuit `having means for compensating-for various engine speeds to provide a substantially constant amplitude sweep throughout the range of `engine speed variations.

The present invention provides a novel constant amplitude sweep generator having means for synchronizingthe sweep frequency with engine speed or with some other recurring phenomenon, and the circuit is particularly advantageous` in that` only about half the number of vacuum tubes are required as are required in other circuits of this type which are known to applicants. Therefore the powerrequirements of the circuit disclosed herein are `much less thanV has in the past been necessary, and the apparatus is particularly adapted for use as amobile unit in a vehicle where the available'power is limited. In the circuit illustrated and described herein, only six tubes arerequired, and, as will vention is that it provides a sweep generator hav'- ing lowspower requirements so that the generator 11 Claims. (C1. 25th-27.)

is particularly adapted for use as a mobile unit, as in testing an automobile engine under road operating conditions; a further feature of the invention is that the circuit includes a relayhaving normally open contacts connected to the sweep generating condenser, and means are provided for briefly energizing the relay to close the contacts and discharge the condenser; stillanother feature ofthe invention is that the circuit includes a single cycle multivibrator having a normally non-conductive tube connected .to the source of triggering pulses, the relay coil being connected in the anode circuitv of` said tube for brief energization upon the occurrence of said pulses; still a Jfurther feature ofthe invention is that a control condenser is provided together with means for charging said condenser to a voltage amplitude dependent upon the repetition rate of the triggering pulses, the voltage of the control condenser being used to regulate the charging rate of the sweep generating condenser as a function of the repetition rate of said triggering pulses; still another feature ofthe inventionis that a` second control condenser is provided and is connected to the first control condenser through rectifying means which permit substantial current flow only in the direction ofthe second control condenser; and an additional feature of the invention is that a low pass filter is connected to the second control condenser for providing a smooth control voltage for regulating the charging rate of the sweep generating condenser.

Other features and advantages will be apparent fromthe following description, and from the drawing which is a schematic diagram of an operative circuit `incorporating theV invention.

Inthe drawing the various operating sections of the circuit are` separated by broken lines. Pulses having a repetition rate dependent upon the speed of the engine under test are obtained from the Vengine under test and are applied to the input circuit of a pulseA clipper and Shaper, and the triggering pulses developed in this circuit are used to trigger a single cycle multivibrator which controls a relay. Brief energization of the relay upon the `occurrence of each triggering pulse causes the normally open contacts of the relay to close briefly so that the sweep generating condenser which is connected to these contacts discharges at the repetition rate of the pulses to synchronize the sweep with engine speed. The closure of the contacts also causes` the discharge of a lirst control condenser at the repetition rate ofthe triggering pulses.4 This control condenser is connected 'to a second control condenser minal VI being connected to the control grid of a triode 3 and terminal 2 being connected to a common lead 4, which may be grounded if desired. The control grid of the triode 3 is connected to ground through a grid leak resistor 5, which may` have a value of 240,000 ohms. The cathode of the triode 3 is connected between the B-plus supply (here shown as a battery' 0 which may have an output of 300 volts) and ground through a voltage divider comprising a resistor l, which may have a value of 150,000 ohms, and an adjustable resistor 8, which may have a maximum value of 10,000 ohms. The cathode resistance portion 8 of the voltage divider is by-passed in conventional manner by a condenser 0, which may have a value of .001 microfarad. The anode of the triode 3 is connected to B-plus through a resistor I0, which may have a value of 150,000 ohms, and the signal output of the output triode 3 is coupled through a condenser II, which may have a value of .001

microiarad, to the control grid of a triode I2. The control grid of the triode I2 is also connected to B-plus through a resistor` I3, which may have a value 'of l megohm; the cathode is connected directly to ground; and the anode is connected to B-plus through a resistor It, which may have a value of 150,000 ohms. i Y

The triodes 3 andv l2 comprise a pulse clipper and shaper, the output pulses of which are of substantially constant amplitude. In the event the source of triggering pulses is a spark pickup associated with the high tension lead of one spark plug of a multi-cylinder engine, the triode 3 acts as a threshold amplifier and eliminates 10W amplitudepulses from other cylinders. Adjustment of the resistor 8 may set the threshold at any desired amplitude.

The recurring voltage pulses developed in the anode circuit of the triode I2 are applied through a coupling condenser I 5, which may have a value of .001 microfarad, to the control grid of the input triode I6 of a single cycle multivibrator comprising triodes I6 and I'I. This multivibrator circuit is so designed that the triode I0 is normally nonconducting, while the triode I'I is normally conducting. A positive bias for the control grid of the triode I6 is provided by a voltage divider comprising resistancesk I8 and I9, which may have values of 300,000 ohms and 100,000 ohms respectively. The anode of the triode I0 is coupled to the control grid of the triode I'I through a condenser which may have a value of .002 microfarad, and a positive bias for the control grid of triode .l 'I is obtained through a B-plus connection including a resistor 2l, which may have a value of 1 megohm. The anode of the triode I'I is connected to B-plus through a resistor 22, which may have a value of' 10,000 ohms. The cathodes of triodes I0 and I'I are connected together, and a common connection is made to ground through a resistor 23, which may have a value of 12,000

ohms.

The anode of the normally non-conductive triode I6 is connected to B-plus through a circuit including resistor l20,which may have a value of 22,000 ohms, connected in series with the operatd ing coil 25a of a relay 25 having stationary contacts 25h and 25e each adapted to close With a movable contact 25d. The contacts are normally open. The stationary contact 25e is connected by means of a lead 20 to one side of a first control condenser'Zl, which,` may 'have' a value of 1 microfarad, and the stationary contact 25h is connected by means of a lead 28 to ground. The leads 26 and 28 respectively incorporate resistors 20 and 30 which may each have a value of 25 ohms and which limit the current iicwing through the relay contacts to prevent welding of the contacts. The movable contact 25d is connected through a lead 3| to a lead 32 which in turn is connected to the ungrounded side of a sweep generating condenser 33, which may have a value of 40 microfarads.

The sweep generating condenser 33 is connected to a charging circuit including the battery 6, a resistor 34, which may have a value of 25,000 ohms, and current control means comprising a pentode 35. The cathode of the tube 35 is connected directly to the sweep generating condenser 33 `as illustrated, and the -suppressor grid of the tube is connected to the cathode. rI'he screen gridof the tube is connected to B-plus by means of a voltage divider comprising resistors 36 and 3l having values of 40,000 and 80,000 ohms respectively. A screen by-pass to ground isprovided by a condenser 38 which may have a value of 16 microfarads.

As is well understood in the art, a` pentode tube is a constant current device Ysince current flow through the tube is substantially independent of the anode voltage thereof throughout a wide range. Consequently, the condenser 33 charges at a substantially constant rate through the tube 35 to provide a linear sweep. When a positive pulse from the pulse clipper and shaper circuit is applied to the grid of the tube i6, this tube is rendered brieiy conductive as the single cycle multivibrator goes through a cycle ofoperation. During thebrief time which tube I0 conducts, relay coil 25a is energized and the contacts of the relay are closed. Closure of contacts 25h and 25d causes the sweep generating condenser 33 to discharge through leads 32 and SI contacts 25d, 25h', andlead 28- to ground. When the multivibrator completes its cycle of operation, tube I6 again becomes non-conductive, the relay coil is deener- Y gized and the relay contacts open, disconnecting the condenser discharge circuit so that the condenser again starts to charge at a linear rate through the tube 35. As is well understood in the art, the condenser does not charge fully during each cycle of operation, and the sweep voltage represents only a small portion of the charging curve` of the condenser.` The output sweep voltage from the condenser 33 may be taken from output terminals 39 and 40 and applied to the horizontal deflection plates. of an oscilloscope. Controlling the triggering of the sweep :generating condenser by means of a sensitive, fast operV ating relay cuts down the number of vacuum tubes utilizedvin the circuit and minimizes .the power requirements of the circuit, making it Vmore advantageous for use as amobile unit..

In orderto provide a sweep of constant amplitude at different engine speeds, Ia controlV voltage circuit includingA the condenser 21 is utilized. The condenser 2l Vis connected into. a. charging circuit comprising a battery IH and aVv resistor or potentiometer 42'. The battery 4I' mayihave output of 45 volts, and hesite negative terminal 75. connected to theV potentiometer 42, which may have a value of 100,000 ohms. A condenser 43 is connected between the positive` terminal of the battery and the movable potentiometer tap to aid in obtaining a constant amplitude sweep by improving the control of the peak voltage developed across condenserZ'l throughout the range of engine speed. The negative side of condenser 21 is connected to relay contact 25e and the positive side is connected to relay contact 25d through leads 32 and 3|, so that this condenser is also discharged when the relay is energized. As in the case of the sweep generating condenser, the control condenser `2'! charges only partially during each cycle, and the peak volta-ge developed there'across at the time of discharge is a function of the repetition rate of the triggering pulses. The negative side of condenser 21 is also connected to the cathode of a diode rectier d4, the anode of which is connected to one side of a second control condenser 45, the other side of which is connected to the lead 32. The anode of the diode 4d is also connected to a low-pass filter comprising resistors 46, 4l, 48 and 49 and condensers 50 and 5I. These resistors may have respective values of 500,000 ohms, 500,000 ohms, 2.8 megohms Iand 3.4 megohms. The condensers 45, 50 and 5l may each have a value of .25 microfarad. The mid-point between resistors 48 and 49 is connected to the-control grid of the tube 35. i 4

In the operation of the control circuit, the condenser 2l partially charges from the battery lil` and it is discharged upon energization of the relay 25. T-hefamplitude of the charge varies directly with the'rate of repetition of the triggering pulses. Because thenegativeside of this condenser is connected to the cathode of the diode '44, said cathode is negative with respect to the diode anode during the charging cycle and current ows through the diode to charge the condenser 45 to the peak voltage of the condenser 21. However, upon discharge of the condenser 2'! the diode isolates the condenser 45 from the discharge circuit so that the peak voltage is maintained across the condenser 45 and is applied to the control grid of the tube 35 to regulate the rate of charge of the sweep generator condenser 33.

While the rectifier d4 is herein illustrated as a diode, it will be obvious that a cold rectifier may be substituted in place thereof further to reduce the number of tubes and the power requirements of the circuit. However, 'a diode is advantageous in that it permits no reverse current flow and has almost negligible resistance to current flow toward the condenser 55. 0n the other hand, many cold rectiers have a resistance of the order of 1000 ohms in the direction of current flow and 100,000 ohms in the reverse direction. Such `anarrangement would add to the ripp-le voltage. Even in the circuit illustrated some ripple is present, although this ripple is greatly reduced by the low-pass filter. The filter is constructed to .have a time constant which is several times .as long as the repetition rate of the triggering pulses at the slowest engine speed so that the control bias voltage at the gridk of `the tube 35 is relatively free from cyclic variation and issub- .stanti'ally constant.` However, when the sweep rate changes the control voltage stabilizes within a few sweep periods.

Obviously the circuit is so designed that neither the condenser 33, the condenser 21, nor the con- -denser 45 charges fully between triggering pulses.

The condensers 33 and 21 are discharged when they are only partially charged so that voltages obtained there'across are substantially linear.

While we have shown and described one embodiment of our invention, it is subject to many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit` and scope of the invention as set forth inthe appended claims.

We claim: Y

l. A constant Iamplitude sweep generator of the character described, including: a sweep generating condenser; a control condenser; charging circuit means connected to said condensers; means V for developing recurring triggering voltage pulses;

means for discharging both said condensers upon the occurrence of said pulses; and a circuit connecting said'control condenser to the sweep generating condenser charging circuit means to regulate the charging rate of said sweep generating condenser as a function of the repetition rate of said pulses.

2. A variable frequency sweep generator of the character described, including: a sweep generating condenser; a charging circuit connected to said condenser; a constant current device in said circuit; a `control condenser; a charging circuit connectedto said control condenser; meansy for developing recurring triggering voltage pulses of variable frequency; means for discharging both said condensers upon the occurrence of said pulses; and a circuit connecting said control condenser to the sweep generating condenser charging circuit means to regulate the charging rate of said sweep generating condenser asa function of the repetition rate ofv said pulses;` Y

3. A constant amplitude sweep-generator of the character described, including: a sweep generating condenser; a` charging circuit connected .to said condenser; current control means in said circuit; means, for developing recurring triggering voltage pulses; a control condenser; a charging circuit connected to said control condenser; means for discharging both of said condensers before they are fully charged upon the occurrence of Vsaid pulses; and a circuit connecting said control condenser to said current control means to regulate the charging rate of said sweep generating condenser as a function of the repetition rate of v said pulses.

4. A constant amplitude sweep generator of the character described, including: a sweep generating condenser; a charging circuit connected to said condenser; current control means in said circuit; means for developing recurring triggering voltage pulses; a first control condenser; a charging circuit c-onnected to said control condenser; means for discharging both of said condensers before they are fully charged upon the occurrence of said pulses; a second control condenser; rectifying means connected between said first and i second control condensers, said rectifying means for discharging both of said condensers before they are fully charged upon theoccurrence of said pulses; a second control condenser; rectifying Vmeans -connected between said first and second 5 wherein said lter'has a time constant which is long compared with the repetition rate of said pulses.

'7. A constant amplitude sweep generator ofthe character described, including: a sweep generating condenser; a charging circuit connected to said condenser; a tube in said circuit having a control element; means for developing recurring triggering voltage pulses; a rst control condenser; a

charging circuit connected to said control condenser; means for discharging both of said condensers before they are fully charged upon the occurrence of said pulses; a second control condenser; rectifying means connected between said first and second control c-ondensers, said rectifying means permitting substantial current flow only in the direction of said second control condenser; a circuit connecting said second control condenser to the control `element of said tube to regulate the charging rate of said sweep generating condenser as a function of the repetition rate of said pulses; and a low-pass filter in said last mentioned circuit, said filter having a time constant which is several times as long as the repetition rate of saidpulses.

8. Apparatus of the. character claimed in claim 1, including control means in said tube whereby the current ow through said tube is substantially independent of the anode voltage thereof throughout a wide range.

9. A variable frequency constant amplitude sweep generator ofthe character described, including: a sweep, generating condenser; a charging circuit connected to said condenser; current control means in said circuit; means for developing recurring triggering voltage pulses; a control condenser; a charging circuit connected to said control condenser; a relay having normally open contacts connected .to said condensers; means for briey energizing said relay upon the occurrence of said pulses to close said contacts and discharge both of said condensers before they are fully charged; and a circuit connecting said control condenser to said current control means to regulate the charging rate of said sweep generating condenser as a function of the repetition rate of said pulses.

10. A variable frequency constant amplitude sweep generator of the character described, including: a sweep generating condenser; a charging circuit connected to said condenser; current 8. control means in said circuit; means for developing recurring triggering voltage pulses; a first control condenser; a charging circuit connected t-o said control condenser; a relay having an op- 51 erating coil and having normally open contacts connected to said condensers; electronic switching means connected to said pulse developing means for briefly energizing said relay upon the occurrence of said lpulses to close said contacts lOY anddischarge both of said condensers before they are fully charged; a second control condenser; rectifying means connected between said first and second control condensers, said rectifying means permitting substantial current flow only in the 1.5i direction of said second control condenser; and

a circuit connecting said second control condenser to said current control means to regulate the charging rate of said sweep generating condenser as a function of the repetition -rate of said pulses.

2m 11. A variable frequency constant amplitude sweep generator of the character described, including: a sweep generating condenser; a charging circuit connected to said condenser; a tube in said circuit having a control element, said tube 25. having the characteristic that the current flow therethrough is substantially independent of the anode voltage thereof throughout a wide range; means for developing recurring triggering voltage pulses; alirst control condenser; a charging cir- 390 cuit connected to said control condenser; a relay 3.5 ing means, said coil being connected-in the anode circuit of said tube for brief energization upon the occurrence of said pulses to close said contacts and discharge both of said condensers after they are only partially charged; a second control condenser; rectifying means connected between said first and second control condensers, said rectifying means permitting substantial current flow only in the direction of said second control condenser; a circuit connecting said second control condenser to the control element of said tube to i regulate the charging rate of said sweep generating condenser as a function of the repetition rate of said pulses; and a low-pass filter in said last mentioned circuit, said filter having a time constant which is several times as long as the repe- 0' tition rate of said pulses.

EDWARD F. WEILER, JR. RICHARD G. DOYEN.

55 V References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,410,920 Atwood Nov. 12, 1946 60 2,489,312 Pacini Nov. 29, 1949 2,519,413 Taylor Aug. 22, 1950 2,554,172 Custin May 22, 1951 2,562,188 `Ilan`ceY July 31, 1951 

